1. Field of the Invention
This invention is in the field of low noise signal cables, particularly audio cables used in a high noise environment such as an automobile or a house equipped with typical appliances and accessories.
2. Description of the Prior Art
As the number of electrical accessories increase, our work, transportation, and living environments are becoming increasingly exposed to unwanted noise. In many situations, the noise from the environment can affect our audio, telephone, or computer by adding noise. This invention describes a new method to reduce the level of noise.
Noisy Listening Environments
In mobile electrical systems, the car""s alternator supplies all the energy for the electrical accessoriesxe2x80x94including the energy to recharge the battery. The car""s conductive chassis is used as a ground return for virtually all of these electrical accessoriesxe2x80x94including the battery. The output of the car""s alternator contains AC (alternating current) ripple that is superimposed upon the DC (direct current) that is used to operate the electrical accessories as well as recharge the battery. This AC by-product of the alternator is therefore conducted over the accessory wires, as well as over the car""s chassis.
Home and pro-audio environments also experience noise from electrical conduits, light dimmers, as-well as other 50 and 60 Hz AC power and control systems. Computer and telephone systems are also prone to noise from electrical systems.
For car audio and other sensitive automotive electrical systems, the AC flowing on the car""s chassis and power distribution system can manifest itself as interference or noise. This means that when the car""s electrical accessories (electric mirrors, rear window de-fogger, engine fan, air conditioning system, electric seats, windshield wiper motors, etc.) are activated, the DC component is used to power the accessories, however, AC interference from the alternator flows on the car""s chassis as well as the accessory power and control wires.
The nature of this type of interference in the car audio, home audio, pro-audio, computer and telephone systems is low frequency because it lies in the 0 to 100,000 Hz bandwidth. Human hearing is generally accepted to lie in the 20 Hz to 20,000 Hz bandwidth. Therefore, the interference that is concentrated between 20 Hz to 20,000 Hz is particularly bothersome because this interference can be superimposed over the audio.
Low frequency accessory noise is coupled into the audio, telephone, computer, etc., components when their signal, power, and/or control cables are placed within the changing electromagnetic field caused by a changing electromagnetic field. For car audio systems, the changing electromagnetic field extends from the case of the alternator to every point on the car""s chassis. Likewise, activating brake light, electric motors, fans, windshield wiper motors, and other accessories can create changing electromagnetic fields. These changing electromagnetic fields extend throughout the accessory power distribution system and control leads as well as over the accessories themselves.
In home, pro-audio, telephone, computer, etc., systems, sources of changing electromagnetic fields are wires, circuits, conduits, and other electrical components.
By a process called induction, a copy of the changing electromagnetic field is induced onto the signal, control and power cables of the audio components. This replica of the changing electromagnetic field is called noise or interference. The interference manifests itself as an audible whining, whirring, popping, or buzzing in the receiving system.
Likewise, the changing electromagnetic field in an accessory circuit can affect sensitive automotive, home, telephone or computer circuits. For instance, the power, control, or signal leads of a car audio component can interfere with control, safety, maintenance, or accessory circuits of the automobile.
For car audio systems, it is common for the deck (i.e., an AM/FM/CD player) to be located in the dash at the front of the car. Other components such as the equalizer(s), electronic crossover(s), processor(s), and amplifier(s) may be located in the console, under seats, on the rear package tray, in the trunk and other places within the vehicle.
The audio bandwidth electrical signal is typically routed from the output of the deck, into the processor(s), the amplifier(s) and finally the loudspeakers. For home, pro-audio, computer, telephone, etc. systems, the components can be separated over great distances.
In all cases, a small AC signal is routed alone a closed loop between the components. Signal cables, phone lines, RCA cables, RCA signal cables, RCA phono cables, parallel conductor cables, co-axial cables, DIN cables, shielded twisted paired cables (STP) and unshielded twisted paired cables (UTP) are used to convey the signal between components. These cables vary in length from a couple of inches up to 20 feet or more. With multi-channel systems, these signal cables carry 2, 3, 4, 5, 6 and more cable pairs.
As seen in FIG. 1, considering an area A placed within a changing magnetic field, the magnetic induction surrounding a wire or current loop inside that field will have closed lines of magnetic induction.
Faraday""s law of electromagnetic induction says that an EMF (electro motive force) appears in a circuit whenever there is a change in the magnetic flux through that circuit. The size or magnitude of the EMF is equal to the flux""s time rate of change through the circuit. Lenz""s law states that the effects of a current associated with an induced EMF oppose the action that is causing the induced EMF.
When two circuits are placed next to each other, and current changes in one of them, the flux associated with the current will also change. An EMF will then be induced in the second circuit. By comparing the coefficients of induction of the two circuits, we arrive at a proportionally constant called the coefficient of mutual induction. This mutual inductance is independent of conductor size and is solely a function of the geometry between the two circuits, see FIG. 2.
Reducing the noise at the source is not practical since it requires modifying the car""s electrical circuits or the home""s wiring.
Due to the nature of low frequency electromagnetic interference, shielding accessory signal, control, and power cables from low frequency noise is both difficult and expensive. Shielding the source of the changing electromagnetic field is not practical because the car""s chassis is one of the largest sources. Likewise, shielding a house or a studio is not practical. Shielding the receiver is also not practical.
Reducing the loop area of the receiving circuit is of benefit, but since the location of the deck (i.e., AM/FM/CD player) is in the car""s dash and the components are frequently placed elsewhere, there is a limit to this solution. Also, since the signal, control and power cables are usually manufactured in specific lengths, the loop areas cannot always be minimized.
Changing the relative orientation between the source and the receiving circuits is not practical due to the size of the car""s chassis and the availability of cable routing passages and bundles. Also, if the orientation improves for one particular electrical accessory (e.g., the headlights), it may increase the interference when another accessory is activated (e.g., the windshield wiper motors).
Using twisted pair cabling (UTP or STP) in lieu of Delta Factor or parallel pairs on the signal path is of benefit. With twisted pair cabling noise is induced equally onto the two conductors of the closed loop. Any noise picked up in the cable pair can be canceled in the receiving stage. Since it is difficult to maintain shield integrity with STP, the use of UTP is the preferred method of conveying a signal between audio components. When compared to Delta Factor or parallel cables, twisted pair cables have made a significant reduction in the level of noise picked up in receiving systems.
The present invention provides a new method of further reducing the induced noise picked up by a receiver in a noisy environment. The method reduces the magnetic flux by decreasing the mutual inductance between the source and the receiving circuit. This decrease is accomplished by increasing the distance between the source and the receiving circuit.
By minimizing the size of the conductors, and maximizing the distance between the conductor and the outer edge of the spacer material, we arrive at a factor called Delta, which represents a difference in space. The greater the Delta between the conductor and the outer edge of the surrounding non-conducting material, the less noise will be coupled into the cable.
By using a nonconductive material to physically increase the separation between the signal, control or power cabling of the receiving circuit (i.e., the UTP), and the noise source (i.e., the car""s chassis), the interference picked up in the receiving circuit is reduced. The greater the separation between the audio signal cable and the car""s chassis, the less noise is picked up in the car audio system, see FIG. 3.
As seen in FIG. 4, as d, the separation distance between the noise source and the receiving circuit, is increased by a factor N, the noise induced into the receiving circuit is decreased. A prior art placement design with cables placed close to the noise source is shown in FIG. 5. The Delta de-fluxing design of the present invention with cables extended away from the noise source is shown in FIGS. 6 and 7.